The forest stand is described as a set of tree size classes and an understory layer. Within one size class the trees are identical and randomly located in the stand. The crown form of a tree can be ellipsoid or conical or the canopy can be homogenous. SPP reads meteorological data as input, calculates sun elevation and the attenuation of direct and diffuse radiation through the canopy and uses the momentary attenuated radiation at point (x,y,z) inside a crown as input for shoot level photosynthetic model(s), p = p(x,y,z,t). Finally, SPP integrates the photosynthetic production over the whole crown and canopy and over the calculation period (typically one year). The time step is 30, 60, 120, 180 or 240 minutes.
There are different options for estimating shoot photosynthesis in SPP. The photosynthetic models RADI, OPTI, and OPAC are described in Mäkelä et al. (2006). RADI is a simple light response curve, while OPTI is based on optimizing stomatal control with respect to transpiration and photosynthesis (Hari et al. 1986). OPAC includes a description of temperature acclimation that accounts for the annual cycle of photosynthesis, using a dynamic delay model (Mäkelä et al. 2004).
Model RODE is described in Dewar (2002). Model USRADI is just another form of RADI: nonrectangular hyperbola with seasonally changing parameters, described in Kolari et al. (2006). Farquhar model (Farquhar et al. 1980) and its analytical solution can be found in Baldocchi (1994). The instantaneous temperature responses of key parameters Qeff, Jmax and Vcmax describe both low and high temperature decrease in photosynthetic efficiency and are formulated in Appendix 1. The stomatal model used in conjunction with Farquhar model is desribed in Leuning (1995). The temperature acclimation model can also be linked with the Farquhar model.
See Oker-Blom et al. (1989), and Mäkelä et al. (2006). An important concept in the light model is intercepted radiation which is used as the driving force of photosynthesis. Intercepted light is calculated as radiation integrated over all directions multiplied by the light extinction coefficient. The light model was originally formulated for uniform overcast diffuse radiation, although CIE standard overcast is another option.
A simple one-layered water balance model is implemented in SPP which operates at a daily timestep. The canopy throughfall is a linear function of above canopy rain, which is not currently linked to LAI. Drainage is estimated either with a bucket model (drainage is the surplus), or with an exponential model (Porté 1999, from GRAECO, personal communication Alex Bosc, INRA Bordeaux, France). Finally, snow is accumulated when daily average air temperature is below some threshold and melted with at rate depending on temperature when above zero.
The effect of soil water limitation on gas exchange is estimated by default with the "embolism avoidance model" (EAM, Duursma et al. 2008), parameters for which are in both the site parameter file (soil hydraulic conductivity, soil water retention parameters), and the size class-specific model parameter file (minimum water potential, etc.). When using the RODE model, the EAM model is obviously not used.
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